Electrical ballast
An electrical ballast is a device placed in series with a load to limit the amount of current in an electrical circuit.
A familiar and widely used example is the inductive ballast used in fluorescent lamps to limit the current through the tube, which would otherwise rise to a destructive level due to the negative differential resistance of the tube's voltage-current characteristic.
Ballasts vary greatly in complexity. They may be as simple as a resistor, inductor, or capacitor (or a combination of these) wired in series with the lamp; or as complex as the electronic ballasts used in compact fluorescent lamps (CFLs).
Current limiting
An electrical ballast is a device that limits the current through an
Ballasts can also be used simply to limit the current in an ordinary, positive-resistance circuit. Prior to the advent of solid-state ignition, automobile ignition systems commonly included a ballast resistor to regulate the voltage applied to the ignition system.
Resistors
Fixed resistors
For simple, low-powered loads such as a neon lamp, a fixed resistor is commonly used. Because the resistance of the ballast resistor is large it determines the current in the circuit, even in the face of negative resistance introduced by the neon lamp.
Ballast was also a component used in early model
Occasionally, this ballast resistor would fail and the classic symptom of this failure was that the engine ran while being cranked (while the resistor was bypassed) but stalled immediately when cranking ceased (and the resistor was reconnected in the circuit via the ignition switch). Modern electronic ignition systems (those used since the 1980s or late '70s) do not require a ballast resistor as they are flexible enough to operate on the lower cranking voltage or the normal operating voltage.
Another common use of a ballast resistor in the automotive industry is adjusting the ventilation fan speed. The ballast is a fixed resistor with usually two center taps, and the fan speed selector switch is used to bypass portions of the ballast: all of them for full speed, and none for the low speed setting. A very common failure occurs when the fan is being constantly run at the next-to-full speed setting (usually 3 out of 4). This will cause a very short piece of resistor coil to be operated with a relatively high current (up to 10 A), eventually burning it out. This will render the fan unable to run at the reduced speed settings.
In some consumer electronic equipment, notably in
Self-variable resistors
Some ballast resistors have the property of increasing in
This property can lead to more precise current control than merely choosing an appropriate fixed resistor. The power lost in the resistive ballast is also reduced because a smaller portion of the overall power is dropped in the ballast compared to what might be required with a fixed resistor.
Earlier[when?], household clothes dryers sometimes incorporated a germicidal lamp in series with an ordinary incandescent lamp; the incandescent lamp operated as the ballast for the germicidal lamp. A commonly used light in the home in the 1960s in 220–240 V countries was a circular tube ballasted by an under-run regular mains filament lamp. Self ballasted mercury-vapor lamps incorporate ordinary tungsten filaments within the overall envelope of the lamp to act as the ballast, and it supplements the otherwise lacking red area of the light spectrum produced.
Reactive ballasts
An inductor, usually a choke, is very common in line-frequency ballasts to provide the proper starting and operating electrical condition to power a fluorescent lamp or HID lamp. (Because of the use of the inductor, such ballasts are usually called magnetic ballasts.) The inductor has two benefits:
- Its reactance limits the power available to the lamp with only minimal power losses in the inductor
- The voltage spike produced when current through the inductor is rapidly interrupted is used in some circuits to first strike the arc in the lamp.
A disadvantage of the inductor is that current is shifted out of phase with the voltage, producing a poor power factor. In more expensive ballasts, a capacitor is often paired with the inductor to correct the power factor. In autotransformer ballasts that control two or more lamps, line-frequency ballasts commonly use different phase relationships between the multiple lamps. This not only mitigates the flicker of the individual lamps, it also helps maintain a high power factor. These ballasts are often called lead-lag ballasts because the current in one lamp leads the mains phase and the current in the other lamp lags the mains phase.
Note: Most American ballast manufacturers describe some of their ballasts as "NPF" (Short for "Normal Power Factor"), but this is misleading, as power factor can only be high or low, not "normal".
In most 220-240V ballasts, the capacitor isn't incorporated inside the ballast like in North American ballasts, but is wired in parallel or in series with the ballast.
In Europe, and most 220-240 V territories, the line voltage is sufficient to start lamps over 30W with a series inductor. In North America and Japan however, the line voltage (120 V or 100 V respectively) may not be sufficient to start lamps over 30 W with a series inductor, so an autotransformer winding is included in the ballast to step up the voltage. The autotransformer is designed with enough leakage inductance (short-circuit inductance) so that the current is appropriately limited.
Because of the large size inductors and capacitors that must be used as well as the heavy iron core of the inductor, reactive ballasts operated at line frequency tend to be large and heavy. They commonly also produce acoustic noise (line-frequency hum).
Prior to 1980 in the United States, polychlorinated biphenyl (PCB)-based oils were used as an insulating oil in many ballasts to provide cooling and electrical isolation (see Transformer oil).
Electronic ballasts
An electronic ballast uses
Electronic ballasts are often based on switched-mode power supply (SMPS) topology, first rectifying the input power and then chopping it at a high frequency. Advanced electronic ballasts may allow dimming via pulse-width modulation or via changing the frequency to a higher value. Ballasts incorporating a microcontroller (digital ballasts) may offer remote control and monitoring via networks such as LonWorks, Digital Addressable Lighting Interface (DALI), DMX512, Digital Serial Interface (DSI) or simple analog control using a 0-10 V DC brightness control signal. Systems with remote control of light level via a wireless mesh network have been introduced.[5]
Electronic ballasts usually supply power to the lamp at a frequency of 20,000 Hz or higher, rather than the mains frequency of 50 – 60 Hz; this substantially eliminates the stroboscopic effect of flicker, a product of the line frequency associated with fluorescent lighting (see photosensitive epilepsy). The high output frequency of an electronic ballast refreshes the phosphors in a fluorescent lamp so rapidly that there is no perceptible flicker. The flicker index, used for measuring perceptible light modulation, has a range from 0.00 to 1.00, with 0 indicating the lowest possibility of flickering and 1 indicating the highest. Lamps operated on magnetic ballasts have a flicker index between 0.04 and 0.07 while digital ballasts have a flicker index of below 0.01.[6]
Because more gas remains ionized in the arc stream, the lamp operates at about 9% higher efficacy above approximately 10 kHz. Lamp efficiency increases sharply at about 10 kHz and continues to improve until approximately 20 kHz.[7] Electronic ballast retrofits to existing street lights had been tested in some Canadian provinces circa 2012;[8] since then LED retrofits have become more common.
With the higher efficiency of the ballast itself and the higher lamp efficacy at higher frequency, electronic ballasts offer higher system efficacy for low pressure lamps like the fluorescent lamp. For HID lamps, there is no improvement of the lamp efficacy in using higher frequency. More than this: HID lamps like the metal halide lamps and high pressure sodium lamps have reduced reliability when operated at high frequencies in the range of 20 – 200 kHz, due to acoustic resonanace; for these lamps a square wave low frequency current drive is mostly used with frequency in the range of 100 – 400 Hz, with the same advantage of lower light depreciation.
Most newer generation electronic ballasts can operate both high pressure sodium (HPS) lamps as well as metal-halide lamps. The ballast initially works as a starter for the arc by its internal ignitor, supplying a high-voltage impulse and, later, it works as a limiter/regulator of the electric flow inside the circuit. Electronic ballasts also run much cooler and are lighter than their magnetic counterparts.[6]
Fluorescent lamp ballasts
Preheating
This technique uses a combination
Although an inductive pulse makes it more likely that the lamp will start when the starter switch opens, it is not actually necessary. The ballast in such systems can equally be a resistor. A number of fluorescent lamp fittings used a filament lamp as the ballast in the late 1950s through to the 1960s. Special lamps were manufactured that were rated at 170 volts and 120 watts. The lamp had a thermal starter built into the 4 pin base. The power requirements were much larger than using an inductive ballast (though the consumed current was the same), but the warmer light from the lamp type of ballast was often preferred by users particularly in a domestic environment.
Resistive ballasts were the only type that was usable when the only supply available to power the fluorescent lamp was DC. Such fittings used the thermal type of starter (mostly because they had gone out of use long before the glow starter was invented), but it was possible to include a choke in the circuit whose sole purpose was to provide a pulse on opening of the starter switch to improve starting. DC fittings were complicated by the need to reverse the polarity of the supply to the tube each time it started. Failure to do so vastly shortened the life of the tube.
Instant start
An instant start ballast does not preheat the electrodes, instead using a relatively high voltage (~600 V) to initiate the discharge arc. It is the most energy efficient type, but yields the fewest lamp-start cycles, as material is blasted from the surface of the cold electrodes each time the lamp is turned on. Instant-start ballasts are best suited to applications with long duty cycles, where the lamps are not frequently turned on and off. Although these were mostly used in countries with 100-120 volt mains supplies (for lamps of 40 W or above), they were briefly popular in other countries because the lamp started without the flicker of switch start systems. The popularity was short lived because of the short lamp life.
Rapid start
A rapid start ballast always heats the lamp electrodes using the same heating power, before, during and after lamp starting, by using a heating transformer coil. It provides longer lamp life and more cycle life than instant start, but have very high ballast losses compared to other types of ballasts, as the electrodes in each end of the lamp continue to consume heating power as the lamp operates. Again, although popular in the United States and Canada for lamps of 40 W and above, rapid start is sometimes used in other countries particularly where the flicker of switch start systems is undesirable.
Some American electronic fluorescent lamp ballasts which are labeled "Rapid start" are otherwise completely different than the classical American rapid start ballast, because they use resonance to start the lamp and heat the cathodes, and don't supply all the time the same heating power regardless the lamp conditions.
Dimmable ballast
A dimmable ballast is very similar to a rapid start ballast, except that the autotransformer is connected to a dimmer. A quadrac type light dimmer can be used with a dimming ballast, which maintains the heating current while allowing lamp current to be controlled. A resistor of about 10 kΩ is required to be connected in parallel with the fluorescent tube to allow reliable firing of the quadrac at low light levels.
Emergency
An electronic ballast with an integrated battery is designed to provide emergency egress lighting in the event of a power failure (typically less than 2 hours). These can be used as an alternative to egress lighting powered by a back-up electrical generator. However, emergency ballasts require regular testing and have a useful life of 10–12 years.
Hybrid
A hybrid ballast has a magnetic core-and-coil
ANSI ballast factor
For a lighting ballast, the
Ballast triode
Early tube-based color TV sets used a ballast
See also
- Iron-hydrogen resistor
- Sodium lamp
References
- ^ ISBN 978-0750649322.
- ISBN 978-0750670739.
- ISBN 978-9814317009. This source uses the term "absolute negative differential resistance" to refer to active resistance
- ^ "Understanding Transformer Noise" (PDF). federalpacific.com. Federal Pacific. Archived from the original (PDF) on 15 March 2015. Retrieved 8 August 2015.
- ^ "infiNET dimmer datasheet" (PDF). Crestron Electronics, Inc. 9 March 2005. Retrieved 22 July 2013.
- ^ a b Specifier Reports: Electronic Ballasts p.18, National Lighting Product Information Program, Volume 8 Number 1, May 2000. Retrieved 13 May 2013.
- ^ IES Lighting Handbook 1984
- ^ "The City of Calgary - Streetlighting Digital Ballast pilot project". Archived from the original on 2013-07-29. Retrieved 2012-06-23.
- ISBN 0-7381-2601-2, page 83
- ^ ANSI standard C82.13-2002 "Definitions for Fluorescent Lamp Ballasts", page 1
- ^ "Ballast factor". Lawrence Berkeley National Laboratory. Archived from the original on March 19, 2013. Retrieved April 12, 2013.